24o THE THEORY OF RELATIVITY

Now, substituting here for A the matrix (40), p. 144, rei that the transposed matrix A is obtained from A by a m of the sign of /?, and multiplying out the right side, the easily convince himself of the identity of (13) with the trar formulae (loa), in which f) = <r2g. This proves the above f which may be restated as follows :

If we make with regard to any ponderomotive assumptions (A) and (B), or, which is the same assumption

then the corresponding pressures, etc., are transformed to (loa) or (10), with «P —c~g.

It is, of course, an entirely different question whe assumptions are to be considered as universally vali Assumption (B) is the expression of the principle of com energy together with the concepts of its localization an< (A) leads to the principle of conservation of momenfam, ' is a strong tendency among the relativists to retain bo1 principles of classical physics. Thus, M. Abraham involving both principles, in his paper on the electrod; ponderable bodies,* and appeals to this equation e\ theory of gravitation, which does not satisfy the p relativity, while Laue makes of it the basis of the genera of continuous bodies. On the other hand, according to 3N electrodynamics of moving ponderable bodies, the pon force and its activity are expressed by that part of the v F— - lor § t, which is normal to the four-velocity F, matrix

or, which is the same thing, by the physical quaternion